What serve? I dont't find as what aie usabale...
A Runestone is a place to hold runes when they aren't on weapons or armor or other items.
So if you decide that you don't need three throwing axes with +2, Striking, and Flaming runes any more because you just got a Thrower's Bandolier, then you can transfer one of each of the +2, Striking, and Flaming runes onto the bandolier. Then transfer the other two copies of each of those runes onto Runestones in order to sell them.
Or you might find a Runestone with a useful rune as dungeon loot.